Aqueous electrolytic bath for coloring anodic oxide layers on aluminum and aluminum alloy substrates and process for coloring said substrates

ABSTRACT

Coloring aqueous electrolytic baths containing aliphatic aminocarboxylic acids and/or salts thereof in addition to coloring metal salt compounds are used to color anodic oxide layers on aluminum and aluminum alloy substrates by alternating current treatment in said aqueous electrolytic baths.

United States Patent Immel et al.

AQUEOUS ELECTROLYTIC BATH FOR COLORING ANODIC OXIDE LAYERS ON ALUMINUM AND ALUMINUM ALLOY SUBSTRATES AND PROCESS FOR COLORING SAID SUBSTRATES Inventors: Waldemar Immel, Solingen; Lorenz Laser, Langenfeld; Werner Adams, Solingen, all of Germany Assignee: Friedr. Blasberg GmbH & Co. KG.,

Solingen-Merscheid, Germany Filed: Oct. 14, 1971 Appl. No.: 189,386

Foreign Application Priority Data Oct. 16, 1970 Germany P 20 50 870.7

U.S. Cl. 204/58 Int. Cl. C23b 9/02, C23f 5/02 Field of Search 204/58, 35 N References Cited UNITED STATES PATENTS 5/1968 Asada 204/58 11 3,773,631 [451 Nov. 20,1973

3,524,799 8/1970 Dale 204/58 2,231,373 2/1941 Schenk 3,664,932 5/1972 Patric 204/58 3,616,297 10/1971 Cooke et al. 204/58 FOREIGN PATENTS OR APPLICATIONS 381,715 3/1963 Japan 204/58 Primary ExaminerJohn H. Mack Assistant Examiner-R. L. Andrews Attorney-Ralph D. Dinklage and Arnold Sprung [5 7] ABSTRACT 8 Claims, No Drawings AQUEOUS ELECTROLYTIC BATH FOR COLORING ANODIC OXIDE LAYERS ON ALUMINUM AND ALUMINUM ALLOY SUBSTRATES AND PROCESS FOR COLORING SAID SUBSTRATES This invention relates to a bath for coloring oxide layers on aluminum substrates and a process for producing light-fast colored oxide layers on aluminum substrates, more particularly to electrolytic baths for producing light fast colored anodic coatings of alumina on aluminum and aluminum alloys, especially with the use of metal salts as coloring component under the action of alternating current.

it is known that oxide layers produced anodically in acidic baths on metals, especially aluminum, can be colored by alternating-current treatment in electrolytic solutions which contain metal salts. Depending upon the metal salt solution used, it is possibleto obtain various colors. The processes are operated in two stages, Inthe first stage, the surface of the aluminum article is anodized in a manner known per se to coat it with an oxide layer. Processes for forming satisfactory uncolored oxide layers by such anodization are generally known in the art and are widely used. After this oxide layer has been formed, the aluminum workpiece is then subjected to the electrolytic coloring process. according to the present invention. This process involves an alternating-current treatment of the workpiece in a metal salt bath which is weakly acidic in most cases thereby coloring the oxide layer formed in the first stage.

German Patent 741,753 describes a process in which salts of copper, nickel, lead and-silver are used for coloring alumina surfaces and in which an electrode consisting of the same metal dissolved as salt in the electrolyte is used as counterelectrode of the aluminum article having an oxidized surface.

It is known from British Patent 1,022,927 and Canadian Patent 762,911 to color anodizedaluminum surfaces electrochemically by treating the alumina layers with a solution which contains metal salts, e.g. nitrates, sulfates, chlorides, oxalates, acetates, citrates, chromates or phosphates of nickel, cobalt, chromium copper, cadmium, silver, gold, lead or zinc in additionto mineral or carboxylic acids. The two patents referred to above also describe the use of metal compounds which form anions in the electrolyte solution such as vanadates, titanates, tellurites, selenites, permangaw nates, ferrous cyanates and ferric cyanates.

However, the prior art electrolytic baths offer a number of considerable difficulties and disadvantages-for the process step of coloring the oxide layer. Known disadvantages are, for example, the poor penetration in the coloring process and the scaling-off of the anodic oxide layers after the electrolytic coloring by means of alternating current. Additionally, the proposed processes do not permit the development of reproducible colors in many cases and "are applicable only to a restricted color scale.

It is also possible that break-through of the alumina layer occurs, i.e. the coloring metal salt ion is not only anchoredin the microporous structure of-the alu mina but there takes place again an electrolytic attack on the aluminum or aluminum alloy. This results in partial dissolution of the base material. In many cases, this effect is caused by vigorous evolution of hydrogen and the associated substantial variation of the pH of the col-' oring solution used. In many cases, an exactly established pH range must be maintained to achieve colo'ring.

it is an object of:the present invention to make the second process step ofithe two-stage process described above, i.e. the coloring of a preformed alumina layer so safe and reliable that the difficulties encountered up to the present are overcome thereby permitting the reliable and reproducible coloring of preformed colorless oxide layers on aluminum and aluminum alloy substrates.

In accordance with the invention, this object is achieved by the concom'ittant use of a specific class of materials in the coloring electrolytic bath used in the stage of alternating-current treatment. It has been found surprisingly that the difficulties mentioned above are overcome by using aqueous electrolytes which contain aliphatic aminocarboxylic acids and/or their salts in solution in addition to the coloring metal salts.

Accordingly, the invention relates to an aqueous electrolyte bath which contains coloring metal salts to color anodic oxide layers on aluminum and aluminum alloy substrates by alternating-current treatment, the novel baths being characterized in that the electrolytic baths contain aliphatic aminocarboxylic acids and/or salts-thereof in solution in addition to the metal salts.

It is another object of the invention to provide a process for coloring anodic oxide layers on aluminum and aluminum alloy substrates by alternating-current treatment in aqueous electrolytic solutions which contain coloringmetalsalts, the process comprising using electrolytic baths which contain aliphatic aminocarboxylic acids and/or salts thereof in solution in addition to the coloring metal salts. If desired, the electrolytic baths according to the invention may contain other known additives in addition to the coloring metal salts and the aliphatic aminocarboxylic acids and/or salts thereof.

Straight-chain or branched-chain aliphatic aminocarboxylic acids may be used. Preferred are aliphatic aminocarboxylic acids which contain not more than one amino group per carboxyl group. Thus, especially preferred are those aminocarboxylic acids the isoelectric point of'which is not in excess of pH 7 and preferably does not exceed about pH 6.5. Aminocarboxylic acids containing more than one carboxyl group, e.g. two carboxyl groups per amino group are useful for the electrolytic bath and the process of the invention. Thus, the aminocarboxylic acids to be used in accordance with the invention preferably are members of the group of what is known as neutral or acidic amino acids asbeing described, for example, by L.F. Fieser and M. Fieser in Lehrbuch der organischen Chemie, 4th German edition (-1960), page 501.

Naturally, those aminocarboxylic acids which are readily available in practice are particularly useful for carrying outthe process of this invention in practice. In most cases, these aminocarboxylic acids are those which'have a limited number'of carbon atoms, e.g. aminocarboxylic acids having :not more than '1 0 carbon atomsQThe aminocarboxylic acids may be present in the electrolytic baths of the invention and'in' the process of the invention with free, i.e. unsubstituted amino groupbut also with a substituted amino group provided that they have sufficient solubilityin the electrolytic solution.'lt may be advantageous to use the aminocarboxylic acids in the form of salts because salts may bemore readily soluble. Examples of suitable substituted amino groups include alkyl-substituted amino groups. The alkyl-substituted aminocarboxylic acids may, for example, be monoand/or dialkyl-substituted aminocarboxylic acids having alkyl groups containing up to five carbon atoms. Particularly useful from this group are, for example, aminocarboxylic acids containing methylor ethyl-substituted amino groups. Thus, the term amino acid as used within this specification and within the claims includes the corresponding compounds having a free amino group and a substituted amino group. However, water-soluble amino acid salts may also be employed or used in addition to the aminocarboxylic acids. Examples of suitable salts of aminocarboxylic acids include, for example, the alkali metal and/or ammonium salts. However, it is also possible to use corresponding salts of the aminocarboxylic acids which form coloring cations in aqueous electrolytic solutions. These cations include especially the cations of the coloring metal salts described in the prior art literature.

The process may be carried out in the neutral to strongly acidic pH range. In the majority of cases, use is made of baths having a neutral to slightly acidic pH range of about pH 3 to 7. The more strongly acidic pH range, e.g. around or below pH 1, is preferred in special cases if coloring metal salts such as tin salts are used which necessitate such more strongly acidic pH values due to their degree of solubility in the electrolyte. The electrodeposition bath solutions of the invention may be acidifed with mineral acids such as hydrochloric acid, sulfuric acid and phosphoric acid if bath solutions having an acidic or, if desired, strongly acid pH range are desired. The concentration of the concentrated mineral acids may, for example, be as high as 100 grams per liter of electrolyte solution.

In the preferred embodiment, the electrolyte baths used according to the invention for the second step of the coloring process described above contain substantially no further constituents in addition to the coloring metal salt and the amino acid. In particular, it is, therefore, generally unnecessary to use also acids or buffer systems although the additional use of such constituents in the coloring electrolytic baths is not precluded according to the invention.

Substantially no upper limit is set to the concentration of the aliphatic aminocarboxylic acid. A natural limit is set to it by the saturation concentration of the particular aminocarboxylic acid used in the metal salt solution. On the other hand, very small amounts of the aminocarboxylic acid give the effect to be achieved in accordance with the invention with respect to improve ment in coloring.

The aminocarboxylic acid is supposed to have an action like that ofa complexing agent. As the metal is anchored in the alumina layer, the aminocarboxylic acid is liberated and is capable of becoming active again. It is desirable, however, to adapt the content of aminocarboxylic acid to some extent to the metal salt content of the electrolyte bath. If noble metal salts, e.g. gold salts are used as the coloring component, low concentrations of metal salt ranging, for example, between 1 and grams per liter will be used in practice. In this case, very small amounts of the aminocarboxylic acid are sufficient, the lower limit being, for example, 0.l gJliter or preferably at least 0.5 g./liter. However, if higher concentrations of metal salt are used, it is desirable to increase also the content of aminocarboxylic acid correspondingly. For example, if the metal salt content ranges between 50 and g./liter, it is desirable to use at least 5 grams and preferably at least 10 g./liter of the aminocarboxylic acid together with the metal salt. In general, it is preferred to use metal salt concentrations of at least about 2 g./liter. The ranges which are particularly important in practice are those from 2 to 300 g./liter and preferably 2 to g./liter dependent, of course, on the solubility of the particular metal salt. The metal salts used are salts of various metals or metalloids which have been described in the prior art literature. The anions of the salts may be inorganic and/or organic acids. Chlorides are known to be not particularly desirable and tend to pitting in alumina layers so that these salts are not used in general in anodizing technique.

The concentration of the aminocarboxylic acids in the electrolytes of the invention is preferably 2 to 200 g. per liter and more preferably 5 100 g. per liter.

The other conditions of the coloring electrolysis step correspond to the conventional operating directions of comparable processes. After a firmly adhering alumina layer has initially been deposited in a manner known per se on the aluminum workpiece, the second, i.e. coloring electrolysis is carried out under the influence of alternating current using current densities which preferably do not exceed 2 A/sq.dm. and more preferably are lower than 1 A/sq.dm. The voltage desirably ranges between about 5 and 30 v. Suitable counterelectrodes include carbon, graphite, a metal which is insoluble in the electrolyte and/or metals which are soluble in the electrolyte and which give the same ions into solution which are used in the particular electrolyte as coloring components. The temperature of the bath preferably ranges from about 10 to 60C, the range from room temperature to about 50C. being particularly preferred. Agitation of the electrolytic bath during the electrolysis may be desirable to ensure a uniform composition of the bath.

The time of the electrolytic treatment, especially in conformity with the current density, determines the intensity of color or shade. In general, a treatment for 0.5 to 15 minutes and preferably 1 to 12 minutes is sufficient to adjust the achievable shades from delicate to intense coloring. Thus, the optical result'can be influenced especially by varying the time of treatment. In general, each metal salt gives a characteristic shade. However, some of the metal salts, e.g. the iron, cobalt and nickel salts, give similar gradations of shades. For the electrolytic baths of the invention, use may be made, for example, of coloring water-soluble heavy metal salts of organic and/or inorganic acids, heavy metals having atomic numbers of 22 to 30, 34, 47 and 48, 50, 52, 79, and 82 being preferred as coloring metal compounds. However, use may also be made, for example, of metal salt compounds which, in aqueous solutions, form anions which contain the coloring metal such as vanadates, titanates, tellurites, selenites, permanganates, and iron cyanates.

The electrolytes of the present invention have excellent penetration or throwing power. The known edge effect, i.e. discoloration at points or spots promoting the current, does no longer occur. The deposits are homogeneously colored and no break-through of the preformed oxide layer occurs. Thus, it is possible with the electrolytes of the invention to achieve uniform and always reproducible colors and shades with satisfactory penetration or throwing power in most simple manner. An increase in the electrolyte temperature within the range specified above generally results in a further improvement of depth penetration, which can be utilized for particularly heavily structured or profilated surfaces. Depending upon the selection of the metal salt, colors and shades within a wide range are obtainable. For example, it is possible to obtain colors from nickel silver to black with one electrolyte.

The serious scaling-off of the preformed oxide layers encountered in prior art processes, especially when operating with higher current densities in the coloring electrolysis is absent in the process of the invention. A particular advantage of the invention is the simple composition of the coloring electrolytic baths. Measures which are necessary in the prior art processes in many cases such as addition of various ingredients (ammonium sulfate, aluminum sulfate, magnesium sulfate or magnesium borate) can be dispensed'with. However,

the presence of these additives does not interfere withthe use of the electrodeposition baths of the invention.

In addition to aliphatic aminocarboxylic acids and/or salts thereof, it is possible, for example, to add boric acid to the electrolytic bath of the invention. For example, up to about 70 to 80% of the aminocarboxylic acid and/or salts thereof may be replaced by boric acid. Preferably 30 to 65% by weight of the aminocarboxylic acid may be replaced by boric acid. However, it is not possible to replace all of the aminocarboxylic acid and- /or the salts thereof by boric acid because in the absence of it satisfactory penetration and the deep black range of shades are not achieved. The sole use of boric acid results in a much lesser penetration or throwing power, and the range of shades which can be achieved has no shades which are deeper than dark bronze or dark brown.

In the examples which follow, aluminum sheet is subjected to alternating current coloring by means of the electrolytic bath of the invention. The oxide layer on the aluminum sheets has been formed as follows:

Aluminum sheets of the grades A1 99.7 or 99.9 or AlMgSi 0.5 are anodized by means of direct current in a 20% aqueous sulfuric acid solution at room temperature for 30 to 50' minutes.

The invention is further illustrated by the examples given hereafter.

EXAMPLE 1 To color the surface, the aluminum sheet having been superficially anodized in the manner described above is subjected to an alternating current treatment with a counterelectrode of carbon or a metal which is insoluble in the electrolyte and with the use ofa soluble anode which gives the same ions into solution of which the coloring electrolyte consists. In all examples, approximately the same conditions were used. It is desirable, as generally known for anodizing baths, to use voltage and/or current regulators which may be controlled by a timer.

The bath contained the following constituents:

80 g./liter of nickel acetate g./liter of glycocoll CH (NH )COOH (glycine) The bath voltage was 12 v., the current density 0.3 to 0.5 A/sq.dm., and the bath temperature C.

The anodized aluminum sheet was treated in this bath for different periods of time. The following range of shades or colors was obtained in dependence upon the time:

1 minute light bronze 3 minutes medium-bronze 5 minutes dark bronze 10 minutes black The colored surface of the sheet shows homogeneous EXAMPLE 2 g./liter of nickel acetate 10 g./liter of aspartic acid Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 3 minutes medium bronze 5 minutes dark bronze 10 minutes black EXAMPLE 3 80 g./liter of nickel acetate 20 g./liter of glutamic acid Test conditions:

0.6 A/sq.dm.

Shades:

1 minute light bronze 3 minutes medium bronze 5 minutes dark bronze 10 minutes black EXAMPLE 4 6O g./liter of nickel acetate 15 g./liter of valine Test conditions:

0.-5 A/sq.dm.

Shades:

1 minute light bronze 3 minutes medium bronze 5 minutes dark bronze 10 minutes black EXAMPLE 5 50 g./liter of nickel sulfate 30 g./liter of glycocoll Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 3 minutes medium bronze 5 minutes dark bronze EXAMPLE 6 80 g./liter of cobalt acetate 20 g./]iter of glycocoll Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 3 minutes medium bronze 5 minutes dark bronze minutes black EXAMPLE 7 60 g./liter of cobalt acetate 10 g./liter of sarcosine Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 3 minutes medium bronze 5 minutes dark bronze 10 minutes black EXAMPLE 8 50 g./liter of copper sulfate 10 g./liter of glycocoll Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light brown 3 minutes medium brown 5 minutes dark brown 8 minutes dark bronze EXAMPLE 9 15 g./1iter of copper sulfate 15 g./liter of glycocoll Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light brown 3 minutes medium brown 5 minutes dark brown 8 minutes dark bronze 10 minutes black EXAMPLE l0 2 g./liter of silver nitrate l5 g./liter of glycocoll Test conditions:

0.5 A/sq.dm.

Shades:

20 seconds light yellow 30 seconds yellow 1 minute light brown 3 minutes red-brown 4 minutes black EXAMPLE 11 1.5 g./liter of gold cyanide l5 g./liter of glycocoll Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light pink 3 minutes pink 5 minutes lilac EXAMPLE 12 1.5 g./1iter of gold chloride 15 g./liter of glycocoll Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light pink 3 minutes pink 5 minutes lilac EXAMPLE 13 1.5 g./liter of gold cyanide 15 g./liter of sarcosine Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light pink 3 minutes pink 5 minutes lilac EXAMPLE 14 EXAMPLE 15 g./1iter of nickel acetate 15 g./liter of alanine Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 3 minutesmedium bronze 5 minutes dark bronze 10 minutes black EXAMPLE 16 60 g./liter of nickel acetate 15 g./liter of DL-aminobutyric acid Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 3 minutes medium bronze 5 minutes dark bronze minutes black EXAMPLE 17 80 g./liter of nickel sulfate g./liter of leucine Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 3 minutes medium bronze 5 minutes dark bronze 10 minutes black EXAMPLE 1:8

20 g./liter of tin sulfate g./liter of concentrated sulfuric acid 2 g./liter of glycocoll- Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 2 minutes medium bronze 5 minutes dark bronze 10 minutes black EXAMPLE 19 g./liter of tin sulfate 80 g./liter of concentrated sulfuric acid 20 g./liter of glycocoll' Test conditions:

0.5 A/sq.dm.

Shades:

1 minutes light bronze 2 minutes medium bronze 5 minutes dark bronze 10 minutes black EXAMPLE 20 50 g./liter of nickel sulfate 4 g./liter of tin sulfate 10 g./liter of conc. sulfuric acid 5 g./liter of glycocoll Test conditions:

0.5 A/sq.dm.

Shades:

1 minute light bronze 2 minutes medium bronze 5 minutes dark bronze 10 minutes black:

EXAMPLE 2] g./liter of nickel sulfate 5 g./liter of glycocoll 5 g./liter of boric acid Test conditions:

0.5 A/sq.dm. 20C.

Shades: h l minute light bronze 3 minutes medium bronze 5 minutes dark bronze 10 minutes black g./liter of cobalt acetate 10' g lliter of aspartie' acid 20' g./lite'r of boric acid- Test conditions:

12 v. r I A Shades:

l= minute light bronze 3' minutes medium bronze 5 minutes dark bronze li0 minutes black EXAMPLE 23' EXAMPLE 24 2 g.-/liter of silver nitrate l0 g.'/liter.'of glycocoll' 5 gJliterofboric acid Test conditions:

Shades:

20 seconds light yellow 30 secondsyellow l minute' light'brown 3 'minutes red-brown 4 minutes black EXAMPLE 25 20 g,./liter-of'tin chloride 20 g;/liter :of :glycocoll 10 g./liter of boric acid Test conditions:

Shjades:

l-minute light bronze 3 minutes medium bronze S -minutes dark'bronze 2 EXAMPLE 22 -1..v

minutes black What is claimed is:

1. In a process for coloring aluminum oxide layers on aluminum and aluminum alloys wherein aluminum is initially anodized in an electrolyte and is thereafter subjected to an alternating current in an electrolyte bath containing coloring metal salts, the improvement which comprises including in said electrolyte bath containing coloring metal salts at least one aliphatic amino carboxylic acid or salt or mixture thereof containing up to l0 carbon atoms in an amount of at least 0.1 gram per liter of said electrolyte containing metal salts.

2. An improvement according to claim 1 wherein the amino acid is present in an amount up to the saturation limit of the electrolyte bath.

3. An improvement according to claim 2 wherein at least one amino acid is present in an amount of between 2 and 200 g per liter in the electrolyte bath.

4. An improvement according to claim 3 wherein the amino acid is present in an amount between 5 and g/liter in the electrolyte bath.

5. An improvement according to claim 1 wherein said aminocarboxylic acid has an isoelectric point not in excess of pH 7.

6. An improvement according to claim 1 wherein the isoelectric point of the amino acid is not higher than about pH 6.5.

7. An improvement according to claim 1 wherein the coloring metal salts are water soluble coloring heavy metal salts of an organic or inorganic acid.

8. An improvement according to claim 1 wherein in the electrolyte bath containing coloring metal salts there is present at least one inorganic acid.

k f t i l P UNITED STATES PATENT OFFICE Tm CERTIFICATE OF CORRECTEON Patent No. 3 .773 .631 Dated November 20 v.

Inventor(s) wa'ldema r Inmei and Lor enz Laser rror appeaiis in the above-identified patent It is certified that eby corrected as shown below:

and that said Letters Patent are her Col. 4,' line 19 After "5" inser t: t6 I Signed and seald this 14pm daypf Ma wm'. 1

(SEAL) Attest v EDWARD M.I+'LETCIER',JR. v c. MARSHALL 'DANN v Attesting Officer Comissioner of Patents 

2. An improvement according to claim 1 wherein the amino acid is present in an amount up to the saturation limit of the electrolyte bath.
 3. An improvement according to claim 2 wherein at least one amino acid is present in an amount of between 2 and 200 g per liter in the electrolyte bath.
 4. An improvement according to claim 3 wherein the amino acid is present in an amount between 5 and 100 g/liter in the electrolyte bath.
 5. An improvement according to claim 1 wherein said aminocarboxylic acid has an isoelectric point not in excess of pH
 7. 6. An improvement according to claim 1 wherein the isoelectric point of the amino acid is not higher than about pH 6.5.
 7. An improvement according to claim 1 wherein the coloring metal salts are water soluble coloring heavy metal salts of an organic or inorganic acid.
 8. An improvement according to claim 1 wherein in the electrolyte bath containing coloring metal salts there is present at least one inorganic acid. 